FIELD OF THE INVENTION
This invention relates to improvements in pipelined computer processors that execute relatively simple instructions in hardware controlled execution units and execute relatively complex instructions in a milli-mode architected state with vertical microcode (i.e. millicode) routines executing in the same hardware controlled execution units. More particularly, this invention relates to an improved read only store as part of cache store for storing frequently used millicode instructions; a read only store which can be simply addressed using a single instruction fetch mechanism for all system instructions.